As is well known, it is necessary in order to ensure stable storage and disposal of industrial wastes, such as radioactive wastes, generated in radioactive substance handling facilities, such as a nuclear power plant, that such wastes should be solidified by Packing them together with a solidifying material into a container, thereby preventing hazardous substances, such as radioactive substances, from diffusing into the environment.
Examples of the solidification method include one in which use is made of a solidifying material such as cement, asphalt, thermoplastics, thermosetting plastics, water glass, or the like. Although all of these solidifying materials are satisfactory from the viewpoint of attaining the object of preventing radioactive substances from diffusing into the environment, differences arise in the handling characteristics and the amount of packing of the waste according to the properties of each of them. Among these solidifying materials, cement has advantages that (1) it is an inorganic material and therefore exhibits high fire resistance, (2) it is hardenable at ordinary temperatures, and (3) it is generally used and therefore stable supply thereof is ensured. However, for the following reasons, the cement has a problem that there is a limitation to the amount of packing of the waste.
That is, a solidifying material comprising cement gives rise to voids as the result of shrinkage accompanying hardening. For details of this shrinkage of cement, reference may be made to A. M. Neville: "Properties of Concrete", 1977 published by Pitman Publishing Limited, (translation by Goto and Osaka published by Gihodo Publishing Co., Ltd. on Nov. 30, 1979). According to this literature, hardened cement consists of gel particles of hydrated cement, gel voids (gel pore) constituted of minute gaps formed among said gel particles, and capillary voids (capillary cavity) formed among agglomerates constituted of said gel particles and said gel voids. When water is mixed with cement at a water to cement ratio of 0.5, 60 ml of water and 40 ml of the cement are present in 100 ml of the resultant mixture. With the progress of hardening of the cement, however, the mixture comes to consist of 61.6 ml of hydrated cement gels, 24.0 ml of gel voids, and 14.4 ml of capillary voids. The gel voids are filled with water generally known as "gel water". Under normal conditions, there is no chance for this water to be removed. Consequently, only capillary voids remain as space and account for as much as 14% of the whole. When such voids are present, there occurs a problem that an increase in the amount of packing of the waste is accompanied by an increase in radioactivity leaching rate. Accordingly, known use of a cement solidifying material has had a drawback that the amount of packing of the waste cannot be increased because otherwise the radioactivity leaching rate will increase.
Examples of the radioactive wastes include those obtained in a boiling water reactor power plant (hereinafter referred to as "BWR plant") of which the major components are sodium sulfate and an ion exchange resin. All of these wastes exhibit water absorptivity. That is, sodium sulfate exhibits water absorptivity through formation of a hydrate and subsequent dissolution thereof. On the other hand, the ion exchange resin exhibits water absorptivity since it has hydrophilic ion exchange groups. When radioactive wastes are to be solidified with hydraulic materials, such as cement, the added water is absorbed by the water-absorptive radioactive wastes since they contain water-absorptive substances as described above. Further, when the amount of packing of the waste is large, the fluidity necessary for hardening the cement cannot be maintained, which makes solidification impossible.
In the above-described known cement solidification technique, no attention has been paid to two factors, i.e., capillary voids generated after hardening and viscosity increase of a water/cement mixture caused by water-absorptive radioactive wastes. Consequently, the known technique has had a drawback that the amount of packing of the waste cannot be increased.